This invention concerns control of the location of the p-n junction in a double heterostructure light emitting diode (LED) or the like. In particular, it concerns such an LED with a window layer at the light emitting face where diffusion of dopant from a portion of the window layer is separated from the p-n junction. The invention also concerns an LED made with a p-type substrate.
LEDs have become ubiquitous devices in a broad variety of applications, with hundreds of millions of LEDs being produced annually. A large proportion of these LEDs emit red light. It is desirable, however, to have yellow and green hues as well. A semiconductor material useful for such LEDs comprises a III-V compound, aluminum indium gallium phosphide, AlInGaP, where by varying the composition, the wave length of light emitted by the LED may be varied.
A typical technique for making an AlInGaP double heterostructure LED employs MOCVD (metalo-organic chemical vapor deposition). A substrate of n-type gallium arsenide is commonly employed. A buffer layer with a gradual change in composition from the composition of the substrate to the composition of the active layers is epitaxially grown on the substrate. Three layers of AlInGaP are then epitaxially grown on the buffer layer. The three layers are referred to as the lower cladding layer on the buffer layer, which is typically doped with tellurium to be n-type, an active layer of a different composition which is deposited without appreciable doping, and an upper cladding layer which is typically doped with magnesium or zinc, or sometimes beryllium or carbon.
One problem with this type of structure is that the electrical conductivity of the upper cladding layer (upper referring to the portion of the LED more remote from the "lower" GaAs substrate) is relatively low. It is therefore desirable to epitaxially grow a window layer on top of the upper cladding layer. The window layer has a relatively high conductivity so that current from an opaque electrode on the upper face of the LED spreads laterally so that light is emitted from areas that are not blocked by the electrode. The refractive index of the window is selected so that it enhances the amount of light emitted from the LED. A high conductivity material such as GaP doped with magnesium or zinc to be p-type may be used. Such a window is described in U.S. Pat. No. 5,008,718.
To enhance these effects, it is desirable to have a relatively thick window, for example, from 20 to 100 micrometers. It takes from 1.5 to 8 hours to grow a window of such thickness by VPE at a temperature of 800.degree. C. This may result in substantial diffusion of the dopant from the upper cladding layer and window through the active layer. When a relatively thick window is grown, diffusion of the p-type dopant may extend an appreciable distance into the lower cladding layer.
FIG. 1 illustrates schematically the concentration of dopant (e.g., magnesium) in the cladding layers and active layer after growing windows in the order of 2, 20 and 60 micrometers. For a very thin window, the p-n junction is near the interface of the active layer A and the upper cladding layer UC. For a window of moderate thickness, the p-n junction is near the boundary between the active layer and the lower cladding layer LC. For a thick layer, however, the p-n junction is deep within the lower cladding layer.
The problem with this is illustrated in the schematic illustration of FIG. 2 which shows light output power (in arbitrary units) as a function of the position of the p-n junction. The light output power drops off rapidly in the lower cladding layer since this is an indirect band gap material with few recombination centers, whereas the active layer is a direct band gap material. It is readily apparent that the maximum light output power as a function of current through the LED is achieved when the p-n junction is approximately coincident with the interface between the active and lower cladding layers.
It is therefore desirable to provide a technique whereby the p-n junction can be maintained in this position.